1. Field of the Invention
The present invention relates to a wide-band antenna, and more particularly to a wide-band antenna which is tunable to a plurality of frequency bands.
2. Brief Description of the Related Art
Antennas are used in a wide variety of applications to receive signals. For example, antennas are used to receive and transmit signals for AM/FM radios, short-wave radios, walkie-talkies, television, cordless telephones, etc. Each antenna, however, must be designed to receive signals in a particular frequency band. Conventional antennas typically use a single loading coil in order to uniquely tune to signals within the band for which it is designed to operate. In order for the antenna to receive signals out of the range for which it was constructed for, a different loading coil capable of tuning to a different frequency band must be used. Thus, conventional antennas are limited in that they can only tune to signals within a certain range and additional tuning elements must be carried and used by the antenna in order for it to be able to tune to signals outside of the frequency bands for which they were designed to operate.
Some standardized frequency bands used by antennas include the High Frequency band (HF), the Very High Frequency band (VHF), and the Ultra High Frequency band (UHF). The HF band generally ranges from 3 to 30 MHz. The VHF band generally ranges from 30 to 300 MHz. And, the UHF band generally ranges from 300 to 3000 MHz. Several conventional antennas have been manufactured for tuning to signals within the frequency band of 3 MHz to 50 MHz, which covers the entire HF range and overlaps with the lower frequencies of the VHF band. For example, the MODEL HS-1000.TM., manufactured by HIGH SIERRA ANTENNA.TM. can tune to the frequency from 3.5 MHz to 30 MHz. Also, the BB-3.RTM. antenna, manufactured by T. J. ANTENNA COMPANY.TM., can tune to the frequency band ranging from 3.5 MHz to 54 MHz. However, these antennas cannot tune to any signals in the upper frequency range of the VHF band and to frequencies in the Ultra High Frequency (UHF) band without additional loading coils designed for those frequencies. Two such frequencies would be, for example, 144 MHz and 430 MHz, respectively. Ability to receive these two higher frequencies is often desirable for an antenna since these two frequencies are popular frequencies for radio communications. However, to receive and transmit the signals of these frequencies, an antenna for these frequencies must be additionally prepared or the loading coils replaced on the antenna.
Moreover, the kinds of antennas tunable to the higher frequencies typically use a variable coil and a rod to reach the higher frequencies. The problem with variable coils, however, is the creation of inductance by the unused sections of the variable coil. The inductance tends to shift the tuning frequency. The effect on tuning is much worse in higher frequencies than lower frequencies. Thus, not only is the ease of use of such conventional antennas poor for receiving and transmitting higher frequencies (i. e. resulting in inconvenience for the users of such antennas), proper tuning on higher frequencies often cannot be performed with prior antennas.